The present invention generally relates to a capacitive sensor, and more particularly to a capacitive sensor capable of detecting an adjacent human body.
Entrapment prevention device is conventionally known, for example, by Japanese Laid-open Patent Application No. 2001-32628 (paragraphs [0002] to [0026], and FIG. 1). Entrapment prevention apparatus is employed to prevent the entrapment of a hand or fingers between a moving member such as a power slide door or power window of a vehicle and a stationary member such as a pillar or window frame for receiving the moving member. The entrapment prevention apparatus includes a capacitive sensor for detecting a human body, and stops or moves the moving member in the reverse direction based on the detection signal.
As shown in FIG. 1A, a capacitive sensor generally includes a sensing electrode E1, an earth electrode E2, an insulator In interposed between the sensing electrode E1 and the earth electrode E2, and a coating member surrounding the peripheries of the sensing electrode E1, the earth electrode E2 and the insulator In and made of an insulating material. The capacitive sensor is positioned, for example, at the end surface of the moving member that faces to the stationary member.
In this capacitive sensor, as shown in FIG. 1B, electric charge q is supplied to the sensing electrode E1 through an oscillation circuit and an output amplifier. To be more specific, the sensing electrode E1 and the earth electrode E2 form a capacitor, and electric potential v outputted through a detection amplifier is given by the following equation (1).v=q/(Ca+Cb)  (1)where q is quantity of electricity to be stored in the capacitive sensor (capacitor), Ca is electrostatic capacity of the capacitive sensor, and Cb is electrostatic capacity between the capacitive sensor and the earth.
When a human body approaches the capacitive sensor, the electric potential v varies according to the change in electrostatic capacity between the capacitive sensor and the human body. In this instance, the electric potential v is given by the following equation (2).v=q/(Ca+Cb+Cc)  (2)where each of q, Ca and Cb is the same as that in the equation (1), and Cc is electrostatic capacity between the capacitive sensor and the human body.
Therefore, Cc increases as the human body becomes closer to the capacitive sensor, and the electric potential v outputted through the detection amplifier becomes smaller as a result. The capacitive sensor detects a human body based on a change of the electric potential v.
However, as shown in FIG. 1C, there may be a possibility that due to rainfall a water droplet W sticks around the periphery of the sensing electrode E1 and the earth electrode E2 of the capacitive sensor, for example, on the coating member. Because the capacitive sensor has the earth electrode E2, if a water droplet W sticks to the coating member, the electrostatic capacity Ca of the capacitive sensor increases under the influence of the electrostatic capacity Cw of the water droplet W.
As is apparent from the equation (2), an increase of the electrostatic capacity Ca causes a decrease in the electric potential v of the sensing electrode E1. In other words, if a water droplet sticks to the capacitive sensor, the capacitive sensor operates incorrectly as if it detects a human body.
FIG. 2 shows an example in which the capacitive sensor is arranged at a power slide door of the vehicle. As seen in FIG. 2, some of the conventional vehicles are provided with a so-called power slide door, in which a door drive device 400 having a motor (not shown) as a drive source moves a slide door 200 in the forward and rearward direction to automatically open and close the slide door 200. The conventional door drive device 400 may be classified into two types: one type including a manipulation means provided at the driver seat or near the door handle so that when the user manipulates the manipulation means, the drive source is actuated to automatically open or close the slide door 200; and the other type for automatically closing the slide door 200 after the slide door 200 is manually moved for a certain distance.
Since the door drive device 400 of these types automatically closes the slide door 200, a passenger or a luggage may be caught between the slide door 200 and the pillar 110 or between the slide door 200 and the front door 500. For this reason, the slide door 200 is typically provided with an entrapment prevention apparatus (not shown) using a capacitive sensor 300 to prevent an entrapment of the slide door 200.
FIG. 3 is an enlarged sectional view taken along the line X—X of FIG. 2.
As shown in FIG. 3, the capacitive sensor 300 generally includes a sensing electrode E1, an earth electrode E2, an insulator In interposed between the sensing electrode E1 and the earth electrode E2, and a coating member 310 surrounding the peripheries of the sensing electrode E1, the earth electrode E2 and the insulator In and made of an insulating material.
As seen in FIG. 3, the capacitive sensor 300 is fixed to a flange portion 202 formed at a front side portion (front end 201) of the slide door 200 through a bracket 600. To be more specific, the bracket 600 having an L-shaped section as viewed from the top is fixed to the flange portion 202 at the interior side of the slide door 200, so that the capacitive sensor 300 is fixed to the slide door 200 within the interior of the vehicle cabin R.
However, the conventional capacitive sensor 300 is fixed to the flange portion 202 formed at the front end 201 of the slide door 200 through the bracket 600, leading to a protrusion of the length L, which corresponds to the dimension of the capacitive sensor 300 (sensor protrusion dimension), from the front end 201 of the slide door 200. By this protrusion, when the slide door 200 moves forwardly, the capacitive sensor 300 becomes closer to the front door 500. In the case where the entrapment threshold value of the capacitive sensor 300 is set to be lower in consideration of safety, the capacitive sensor 300 too sensitively detects an object, leading to a detection error of the capacitive sensor 300.
There is also a drawback in that the protruding arrangement of the capacitive sensor 300 from the front end 201 of the slide door 200 detracts from the appearance of the vehicle.
Further, since the capacitive sensor 300 is fixed to the slide door 200 through the bracket 600, fastening members such as screws are required to fix the bracket 600 against the capacitive sensor 300 and the slide door 200. This results in a drawback that the number of parts or the number of manufacturing processes increases.
In one aspect, the present invention seeks to provide a capacitive sensor and an entrapment prevention apparatus which can prevent a detection error of the capacitive sensor caused by a water droplet sticking to the periphery of the sensor.
In another aspect, the present invention also seeks to provide a capacitive sensor which can minimize the sensor protrusion dimension when the capacitive sensor is fixed to a fixing member and which can ease the attachment of the capacitive sensor to the fixing member.